Micromechanical sensors for sensing a physical quantity, such as acceleration, vibration or electrostatic potential, are useful in many applications, including air bag deployment and active suspension in automobiles, and guidance systems in military weapons, among others. A micromechanical sensing apparatus may include a micromechanical sensor in-the form of a suspended microstructure and a circuit responsive to the micromechanical sensor for providing an output representative of a sensed quantity. The suspended microstructure includes stationary and movable elements which are conductive. The micromechanical sensor may be configured for sensing acceleration. When an acceleration sensor of this type is subjected to an accelerative force, the movable element moves relative to the stationary element, producing an electrical output that is sensed by the circuit. The stationary and movable elements form a capacitor which changes in capacitance when the sensor is subjected to an accelerative force. The micromechanical sensor may have a differential capacitor configuration in which one capacitor increases and the other capacitor decreases when the sensor is subjected to an accelerative force.
In certain instances, an actuation signal may be applied to the micromechanical sensor. For example, one way of testing such micromechanical sensing apparatus is by application of an actuation signal, or test signal, to the capacitors. The test signal may, for example, be a pulse of prescribed amplitude. The test signal charges the capacitors, causing electrostatic deflection of the movable element relative to the stationary element. The deflection produces an output from the circuit. One problem that may occur in connection with electrostatic deflection of the movable element by a test signal is that the movable element may come in contact with the stationary element. Such contact is undesirable, since sticking between the contacting surfaces is likely, particularly when one or both of the surfaces is polysilicon. When a portion of the suspended microstructure sticks to another portion of the device, it is very difficult to separate the two, thereby frequently resulting in failure of the device. It is therefore desirable to provide methods and apparatus for electrostatically deflecting the elements of a microstructure with an actuation signal, while reducing or eliminating the risk that the elements will contact each other.
Contact between the stationary and movable elements of a microstructure may result from electrostatic deflection in two related ways. The movable element may be deflected in response to a relatively large actuation signal that is sufficient to bring the movable element in contact with the stationary element. In addition, electrostatic capture may be caused by an AC or DC potential difference (or both together) between the movable element and the stationary element. The force caused by any potential difference is attractive and increases relative to 1/(gap distance).sup.2. Thus, as the stationary and movable elements get closer, the force gets stronger faster than the pulling back tendency of the mechanical spring, which is linear with distance. With very sensitive microstructures that have a weak spring, the movable element does not have to move very far before the attractive forces overcome the restoring force of the spring. In this case, the movable element is drawn into contact with the stationary element and is electrostatically captured. A relatively small actuation signal may deflect the movable element within the electrostatic capture range of the stationary element. It is desirable to prevent contact which may result from a relatively large actuation signal or from electrostatic capture.